Short-Term Debt (Details) (USD $)	Feb. 28, 2013	Aug. 31, 2012	Feb. 29, 2012
Short-term debt
Short-Term Debt	$ 314,328,000	$ 110,640,000	$ 306,095,000
Commodity Credit Corporation (CCC)
Short-term debt
Maximum borrowings	245,100,000
Short-Term Debt	0		0
Seasonal line of credit | CoBank, ACB
Short-term debt
Maximum borrowings	410,000,000
Maximum borrowings	350,000,000
Additional borrowing under line of credit, which can be utilized for either short-term or long-term borrowing purposes	50,000,000
Short-Term Debt	0		0
Amount outstanding	3,200,000		3,100,000
Amount utilized for long-term borrowing purposes	3,200,000		9,000,000
Unused borrowing capacity	140,300,000		82,800,000
Unused borrowing capacity which can be utilized for short-term and long-term borrowings	46,800,000		41,000,000
Short-term line of credit | Wells Fargo Bank
Short-term debt
Maximum borrowings	1,000,000
Short-Term Debt	0		0
Commercial Paper
Short-term debt
Short-Term Debt	$ 314,300,000		$ 306,095,000
Commercial Paper | Minimum
Short-term debt
Fixed interest rate (as a percent)	0.38%		0.42%
Commercial Paper | Maximum
Short-term debt
Fixed interest rate (as a percent)	0.51%		0.57%